This meta-analysis, like almost all others, ignores the vitamin D dose size
Thus the same importance was given as 400 IU daily as 4,000 IU daily

40% reduction for ages 1-11 - average for all dose sizes

Vitamin D must be very powerful if benefits are found even when dose size is ignored

Imagine if Aspirin had a similar meta-analysis, with dose sizes ranging from 40 mg to 400 mg

Most of the loading-dose (Bolus) trials examined used only a single 120,000 IU dose of vitamin D
Most successful adult loading-dose/Bolus treatments use dose sizes of 300,000 IU to 1,500,000 IU of vitamin D

Data sources Medline, Embase, the Cochrane Central Register of Controlled Trials, Web of Science, ClinicalTrials.gov, and the International Standard Randomised Controlled Trials Number registry from inception to December 2015.

Eligibility criteria for study selection Randomised, double blind, placebo controlled trials of supplementation with vitamin D3 or vitamin D2 of any duration were eligible for inclusion if they had been approved by a research ethics committee and if data on incidence of acute respiratory tract infection were collected prospectively and prespecified as an efficacy outcome.

Results 25 eligible randomised controlled trials (total 11 321 participants, aged 0 to 95 years) were identified. IPD were obtained for 10 933 (96.6%) participants.
Vitamin D supplementation [ ignoring dose size] reduced the risk of acute respiratory tract infection among all participants (adjusted odds ratio 0.88, 95% confidence interval 0.81 to 0.96; P for heterogeneity <0.001). In subgroup analysis, protective effects were seen in those receiving daily or weekly vitamin D without additional bolus doses (adjusted odds ratio 0.81, 0.72 to 0.91) but not in those receiving one or more bolus doses (adjusted odds ratio 0.97, 0.86 to 1.10; P for interaction=0.05).
Among those receiving daily or weekly vitamin D, protective effects were stronger in those with baseline 25-hydroxyvitamin D levels <25 nmol/L (adjusted odds ratio 0.30, 0.17 to 0.53) than in those with baseline 25-hydroxyvitamin D levels ≥25 nmol/L (adjusted odds ratio 0.75, 0.60 to 0.95; P for interaction=0.006). Vitamin D did not influence the proportion of participants experiencing at least one serious adverse event (adjusted odds ratio 0.98, 0.80 to 1.20, P=0.83). The body of evidence contributing to these analyses was assessed as being of high quality.

Conclusions Vitamin D supplementation was safe and it protected against acute respiratory tract infection overall. Patients who were very vitamin D deficient and those not receiving bolus doses experienced the most benefit.

Vitamin D deficiency, and the role of vitamin D supplementation, is controversial. However, it is hard to ignore the recent systematic review suggesting that, at last, we may have a magic bullet for reducing the rate of respiratory tract infections in children.[1] This systematic review was unusual in that the authors obtained the individual data from all 10 933 participants in the 25 randomised trials included in their review. Such non-aggregated data enable a far more reliable assessment of response in pre-specified subgroups. The quality of the evidence was rated as high, and the overall result was protective, with an adjusted odds ratio (OR) of 0.88; 95% confidence interval (CI) 0.81, 0.96; P = 0.001; number needed to treat (NNT) = 33 (to prevent one respiratory tract infection).

Twelve randomised controlled trials were in children <16 years;

four in infants <1 year (n = 5571) and

eight in 1.1–15.9 years (n = 1079).

No significant benefit was seen in infants; adjusted OR = 0.94 (95% CI 0.83, 1.06; P = 0.33).
However, in the 1.1–15.9 year age group, the adjusted OR was 0.60 (95% CI 0.46, 0.77; P < 0.001), with an NNT of 8. Subgroup analysis found, unsurprisingly, a better response in those with low baseline vitamin D levels (i.e. 25-hydroxyvitamin D < 25 nmol/L). Surprisingly, daily or weekly supplementation was more effective than bolus injections. Influenza vaccine status had no significant effect on response. The generalisability of these findings is unclear.